JP2015-098802A discloses an engine which introduces internal EGR gas into a combustion chamber within an operating range in which mixture gas in the combustion chamber is combusted by self-ignition. This engine includes a valve operating mechanism for opening an exhaust valve, and an operation mode of the valve operating mechanism is switchable between a normal mode and a special mode. For example, in the normal mode, the exhaust valve is opened on exhaust stroke, and in the special mode, the exhaust valve is opened on the exhaust stroke and intake stroke. The valve operating mechanism has a first cam and a second cam. The first cam is selected in the normal mode and has one cam nose. The second cam is selected in the special mode and has two cam noses. Specifically, the second cam has a cam profile in which, after the exhaust valve is opened, a lift gradually increases to a highest lift and then gradually decreases. This gradual decrease does not bring the lift directly to zero, but maintains the lift at a given lift, which is lower than the highest lift, and then brings it to zero. Thus, in the special mode, the exhaust valve is opened on the intake stroke. In the engine described in JP2015-098802A, when the internal EGR gas is introduced into the combustion chamber, a controller operates the valve operating mechanism of the exhaust valve in the special mode. Thus, the exhaust valve opens on the intake stroke, and a part of exhaust gas discharged to an exhaust port during the exhaust stroke is reintroduced into the combustion chamber. Further, by adjusting an internal exhaust gas recirculation (EGR) ratio (a weight ratio of internal EGR gas to a weight of the entire gas introduced into the combustion chamber), a temperature state inside the combustion chamber is suitably maintained and the mixture gas is stably combusted by self-ignition.
In the engine described in JP2015-098802A, the self-ignition combustion of the mixture gas inside the combustion chamber is limited to be performed within a narrow operating range where an engine load is low. In this regard, a fuel efficiency of the engine improves by extending this operating range to a higher engine load side.
Therefore, a case may be considered that an engine includes a booster for boosting intake air introduced into a combustion chamber, and within a high engine load range, intake air is boosted so that a fresh air amount increases corresponding to an increase of a fuel amount. By this, the operating range in which the mixture gas is combusted inside the combustion chamber by self-ignition is extended to the higher engine load side.
However, when the exhaust valve opens on the intake stroke as described above while the booster boosts the intake air, since both the exhaust valve and the intake valve are opened on the intake stroke, blow-by of the intake air from the intake side to the exhaust side of the engine occurs. Thus, it becomes difficult to accurately adjust the amount of internal EGR gas introduced into the combustion chamber. In other words, the controllability of the internal EGR ratio degrades.